Advancement over the past decade of materials based on two-photon absorption has been phenomenal; yet, a need for greater understanding of multi-photon processes remains. Questions of design to maximize two-photon absorbance cross-sections and quantum yields have been addressed quantitatively. However, effects of the chemical environment on the absorption and emission processes in these chromophores are largely unexplored. This study presents the effects of 26 different solvents on the absorption and emission characteristics of a set of six new alkylamino-substituted styryl pyrazines. The solvatochromic comparison method is employed to derive structure-optical property relationships within this family of fluorophores, and the effects of solvent polarity, hydrogen bonding and inductive and dispersive forces are discussed. Electrostatic and transition moments are measured and their contributions to two-photon absorbance cross-sections (δTPA) are discussed. Symmetric D-π-A-π-D molecular architecture is thought to play a very important role in increasing δTPA by disrupting the degeneracy of excited states and increasing one- and two-photon transition probabilities. The coupling of strong electron donors to strong electron acceptors with symmetric D-π-A-π-D molecular architecture is shown to produce fluorophores with exceptional two-photon activities.
The design, synthesis, and characterization of a series of styryl pyrazine derivatives as chemically responsive fluorophores are reported. These styryl pyrazines were ideal for structure-property relationship studies designed to elucidate the role of molecular symmetry, polarity, planarity and cooperative and competitive intramolecular charge transfer interactions in determining their colorimetric or fluorimetric responses. These fluorophores were designed to exhibit large changes in emission in response to changes in solvent composition or addition of various analyte species. The large solvatochromic and analyte-induced changes in their spectra were related to the nature of molecular polarization upon excitation, as well as stabilization of the excited state by the molecular environment. Several of these molecules shared the structural and electronic features common to quadupolar two-photon chromophores, and were thus expected to function as chemically responsive two-photon fluorophores, as well. Calculations of their second hyperpolarizabilities (γ(-ω;ω,-ω,ω))and comparison to known two-photon molecules showed that these molecules were expected to be exceptional two-photon active molecules.